1. Field of the Invention
The present invention relates to a gas sensor having a protector for protecting, a detecting element exposed to exhaust gas from contact with adhering water droplets.
2. Description of the Related Art
Conventionally, a gas sensor is known having a detecting element in which electromotive force is generated in a magnitude corresponding to the concentration of a specific gas; for example, NOx (nitrogen oxides) or oxygen, contained in exhaust gas of an automobile or the like, or whose resistance varies with the concentration. The gas sensor is attached to an exhaust pipe of an automobile, or the like. Since the detecting element is exposed to exhaust gas of high temperature, subjecting the detecting element to thermal shock caused by water contained in exhaust gas and adhering to the detecting element may crack or break the detecting element. In order to cope with this problem, the gas sensor is fitted with a protector for covering the detecting element, thereby protecting the detecting element from contact with adhering water droplets (refer to, for example, Patent Document 1).
The protector of the gas sensor of Patent Document 1 has a dual structure consisting of an inner protector (inner tubular member) which covers a front end portion of the detecting element projecting from a metallic shell, and an outer protector (outer tubular member) which radially encloses the inner protector so as to form a clearance present between the inner protector and the outer protector. Exhaust gas is introduced into the clearance between the outer protector and the inner protector through outer introduction holes (outer-wall gas introduction holes) formed in the outer protector. The exhaust gas is then introduced into the inner protector through inner introduction holes (inner-wall gas introduction holes) formed in the inner protector, to thereby contact the detecting element.
A front end portion of the inner protector of the gas sensor of Patent Document 1 is tapered and projects from a front end of the outer protector. An exhaust hole is formed in a front end of the inner protector. When exhaust gas flowing outside the gas sensor contacts the taper, a gas flow along the taper is generated, to thereby generate a negative pressure (suction) in the vicinity of the exhaust hole. By virtue of the suction, exhaust gas which has been introduced into the inner protector is promptly discharged through the exhaust hole. Particularly, gas flow along the taper generates suction around a portion of the inner protector and a portion of the outer protector located in the vicinity of the exhaust hole.
[Patent Document 1] Japanese Patent Application Laid-Open (kokai) No. 2004-109125
3. Problems to be Solved by the Invention
However, in the gas sensor of Patent Document 1, the exhaust hole of the inner protector is sized to ensure discharge of exhaust gas. Thus, in some cases, a water droplet or the like could enter the inner protector through the larger-sized exhaust hole. Since a front end portion of the detecting element is directly visible from the outside, a water droplet which enters through the exhaust port may adhere to the detecting element. If the detecting element is subjected to thermal shock caused by an adhering water droplet, the detecting element may crack or break.